Re: [Qemu-devel] [PATCH v3 11/11] translate-all: add tb hash bucket info to 'info jit' dump

[Emilio G. Cota]

On Sun, Apr 24, 2016 at 12:46:08 -0700, Richard Henderson wrote:
> On 04/22/2016 04:57 PM, Emilio G. Cota wrote:
> >On Fri, Apr 22, 2016 at 12:59:52 -0700, Richard Henderson wrote:
> >>FWIW, so that I could get an idea of how the stats change as we improve the
> >>hashing, I inserted the attachment 1 patch between patches 5 and 6, and with
> >>attachment 2 attempting to fix the accounting for patches 9 and 10.
> >
> >For qht, I dislike the approach of reporting "avg chain" per-element,
> >instead of per-bucket. Performance for a bucket whose entries are
> >all valid is virtually the same as that of a bucket that only
> >has one valid element; thus, with per-bucket reporting, we'd say that
> >the chain lenght is 1 in both cases, i.e. "perfect". With per-element
> >reporting, we'd report 4 (on a 64-bit host, since that's the value of
> >QHT_BUCKET_ENTRIES) when the bucket is full, which IMO gives the
> >wrong idea (users would think they're in trouble, when they're not).
> 
> But otherwise you have no way of knowing how full the buckets are.  The
> bucket size is just something that you have to keep in mind.

I'll make some changes in v4 that I think will address both your and
my concerns:
- Report the number of empty buckets
- Do not count empty buckets when reporting avg bucket chain length
- Report average bucket occupancy (in %, so that QHT_BUCKET_ENTRIES
  does not have to be reported.)

> >If those numbers are off, then either this
> >     assert(hinfo.used_entries ==
> >            tcg_ctx.tb_ctx.nb_tbs - tcg_ctx.tb_ctx.tb_phys_invalidate_count);
> >should trigger, or the accounting isn't right.
> 
> I think I used an NDEBUG build, so these weren't effective.
> 
> >Only the second report ("after 7/11") seems good (taking into account
> >lack of precision of just 3 decimals):
> >   5.26*32582=171381.32 ~= 171367
> >which leads me to believe that you've used the TB and invalidate
> >counts from that test.
> 
> The TB and invalidate numbers are repeatable; the same every time.

Then something else is going on, because both the 1st and 3rd tests are
way off. I'd re-test with assertions enabled.

Thanks,

                Emilio